T cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy that accounts for 15% of pediatric and 25% of adult ALL cases. Although the overall survival rates for children with T-ALL have improved, 25% relapse and most succumb to disease. Currently, T-ALL is treated with multi-agent chemotherapy and no targeted therapies exist. T-ALL is largely caused by activation of the TAL1 and NOTCH1 oncogenic pathways. We have shown that Tal1 contributes to leukemia by interfering with E proteins, critical regulators of lymphoid development. We also identified mutations in Notch1 as an important cooperating event and showed that mouse T-ALL growth remains dependent on Notch1. Therefore, both TAL1 and NOTCH1 contribute to T-ALL growth and survival, but it remains unknown whether human T-ALL growth can be limited in vivo by inhibiting only NOTCH1 or whether the TAL1/E2A oncogenic pathway will also need to be targeted. The overall goals of this research program are to understand how the TAL1/E2A and NOTCH1 pathways give rise to T-ALL and to use this mechanistic knowledge to test new therapeutic approaches in our mouse and newly established human T-ALL models. We hypothesize that the high rates of therapeutic relapse observed in T-ALL patients reflects an inability to eliminate leukemia-initiating cells (L-ICs), a rare population of leukemic cells required to initiate and perpetuate disease. Using our mouse T-ALL models, we demonstrate that committed thymic progenitors are enriched in disease potential and that Notch1 inhibition reduces mouse L-IC activity. A goal of this proposal is to purify and characterize the mouse L-IC and to determine how Notch1 mediates L-IC self-renewal and whether Tal1 also contributes (Aim1). Our preliminary studies show that silencing the TAL1 oncogene sensitizes human T-ALL cells to the effects of NOTCH1 inhibition and induces apoptosis. A final objective of this proposal is to develop novel TAL1 inhibitors and to test whether TAL1 and NOTCH1 inhibition is sufficient to eliminate human L-ICs and prolong the survival of immunodeficient NOD-scid IL2R3null mice engrafted with primary pediatric T-ALL cells. PUBLIC HEALTH RELEVANCE: The overall goal of this research program is to understand how the TAL1 and NOTCH1 oncogenes cooperate to cause T cell leukemia. The work proposed will use mouse and human models of the disease to determine whether targeting TAL1 and/or NOTCH1 reduces leukemia initiating cell (L-IC) growth and results in long term survival.